(1) Field of the Invention
The present invention relates to a circuit for generating a discrimination voltage level which is used as a reference voltage level to discriminate between two adjacent logic levels of an input signal which can have a plurality of different voltage levels generally corresponding to a plurality of respective, different logic levels.
Generally, in the field of digital systems, more particularly, in digital communication technology, digital signals can be in a state corresponding to one of two or more predetermined discrete levels, e.g., two-state signals can be in a high level state (hereinafter called H level) or in a low level state (hereinafter called L level).
When digital signals as mentioned above are output from one unit and received by another unit, sometimes it is necessary to recognize the level (or state) of the received signal, for example, in regenerative repeaters located along transmission lines on which digital signals are transmitted, digital signals transmitted on a transmission line and received in the regenerative repeaters are amplified, reshaped, retimed, and retransmitted on the transmission line. To retransmit the received digital signals, the repeaters must know the level of each received signal, i.e., at which level each output signal should be sent out as a regenerated digital signal. For recognizing the level (or state) of the received signal, (voltage level) discrimination circuits are provided, for example, in the regenerative repeaters or other communication system which function to receive digital signals.
A discrimination circuit generally comprises a circuit for generating a discrimination voltage level (V.sub.ref), and the discrimination voltage level (V.sub.ref) is used to discriminate between different levels of the received input signals, for example, in two-state systems, to discriminate between a high level and a low level, i.e., a discrimination circuit outputs a high level signal when the voltage level of an input signal is higher than the discrimination level voltage (V.sub.ref), and outputs a low level signal when the voltage level of an input signal is lower than the discrimination voltage level (V.sub.ref).
The voltage level of a digital signal (electric signal or optical signal), however, fluctuates in accordance with fluctuations of temperature and environmental conditions, and varies with deterioration due to age of the system in which the digital signal is generated and transmitted. Further, noise contributes to the fluctuation and variation of the level of the digital signal, when superposed on the signal, and this also varies with the temperature and environmental conditions, and deterioration due to age.
In particular, a digital signal transmitted over a long transmission line is greatly affected by the fluctuation and variations due to the above conditions, i.e., input signals to repeaters, which are generally located along a long transmission line, may be greatly fluctuated (varied) due to the fluctuations and variations of the above conditions.
Since the fluctuation of the levels of digital input signals causes a high error rate when discriminating between different levels of the input signals in a discrimination circuit, there is a great demand to minimize the effect of the above fluctuations of input signal levels in a discrimination circuit, and accordingly, to improve the reliability of the discrimination circuit, in addition to a general demand for a reduction of costs.
In particular, when used in regenerative repeaters, since the regenerative repeaters are located such that a technician can not obtain access thereto, there is a further demand that a level adjustment or another regulating operation after the repeaters are installed, be eliminated.
(2) Description of the Related Art
In the prior art, to conpensate the effect of temperature fluctuations, a temperature compensation circuit is provided in the circuit for generating a discrimination voltage level. This compensation circuit, for example, comprises a resistor having a value which varies in accordance with ambient temperature, and the discrimination voltage level is obtained from the voltage between the terminals of the resistor, so that the discrimination voltage level can be used to realize an optimum discrimination of different levels of input signals having levels which have been varied due to changes in the ambient temperature.
In the above method using a temperature compensation circuit, however, compensation for a fluctuation factor other than the temperature fluctuation is impossible, and a cumbersome operation of adjusting the initial discrimination level in each circuit for generating a discrimination voltage level must be carried out in accordance with the conditions at the location of the discrimination circuit to which the discrimination voltage level is supplied, and this causes an increase of the manufacturing costs.
Another method of decreasing the error rate in the discrimination of the level of the input signals in the prior art is to increase the system margin, i.e., to increase the noise margin, in a stage before the discrimination circuit, but, it is very expensive to provide the construction for increasing a system margin, e.g., for increasing gains in amplifiers, and the effect of this method is not satisfactory because fluctuation factors other than the fluctuation due to the superposing of noise in the stages where the system margin is increased, are not removed.
A third method of controlling the discrimination voltage level to be supplied to the discrimination circuit in the prior art is to use a data format which periodically includes one or more predetermined bits in the transmitted data, and detects the occurrence of errors by recognizing the predetermined bits using the discrimination voltage level, such as "H".fwdarw."L" (errors by recognizing a high level signal as a low level signal), and "L".fwdarw."H" (errors by recognizing a low level signal as a high level signal), and the discrimination level is controlled based on the result of the detection of the occurrence of errors.
For example, an mBIC code method is known wherein m is a natural number, e.g., when m=10, this is called a 10BIC method. Here, a redundancy bit is inserted into each ten original data bits (i.e., herein the data transfer rate is changed by 11/10 times) to form a block consisting of ten original data bits (1st.about.10th bit in the above block), and one redundancy bit, and the redundancy bit (eleventh bit in the above block) is set as an inverted value of a predetermined one of the bits in the original data, e.g., a bit just before each redundancy bit (and thus the tenth bit in the succession of the original ten data bits). Accordingly, the occurrence of errors can be recognized by monitoring the above tenth and eleventh bits in each block.
In the above third method, however, a limitation is added to the data format, and the data transfer rate must be changed to periodically insert the above predetermined bits in the original transmitted data, and further, an additional construction for periodically detecting the above occurrence of errors, becomes necessary.